The present invention relates generally to equipment for testing physical characteristics of materials, and more specifically, to a column assembly for a test machine that enables the machine to subject a sample of material to high-cycle stress.
High-cycle fatigue (HCF) is so-called because it appears after millions of repetitive cycles of stressful vibrations or other use, which can affect even high-strength metals such as the titanium alloys used in the gas turbines that power aircraft. These materials and components must endure repeated, long-term exposure to high temperatures and static and dynamic stresses caused by rotation speeds of 7,000 to 10,000 revolutions per minute. Under these conditions, blade and disk materials must be able to withstand a large number of stress cycles. Fatigue cracks can initiate and grow at stress levels that are low in relation to the material""s yield stress. And, because of the very large number of cycles involved, laboratory study of HCF failure phenomena must be carried out at high frequencies to characterize the behavior of cracks within reasonable time limits.
U.S. Pat. No. 6,023,980, to Thomas E. Owen, et al., entitled xe2x80x9cHigh-Cycle Fatigue Test Machinexe2x80x9d, describes embodiments of a test machine that may be used to statically and dynamically load a test specimen in a manner that introduces controlled cyclic fatigue forces at high vibrational rates. In one embodiment, the test machine combines a high-frequency machine with a scanning electron microscope (SEM). This SEM-compatible test machine applies stress to a test specimen by two means. A steady stress is applied by hydraulic pressure. At the same time, high-frequency dynamic stresses are applied by piezoceramic actuators that cause the machine as well as the specimen to resonate between 1,000 to 1,700 cycles per second. The resonance condition amplifies the dynamic loading generated by the piezoceramic actuators and is an important design feature of the machine. Static loads of up to 6,000 pounds, and dynamic loads of up to xc2x11,200 pounds, can be applied to a specimen.
In another embodiment described in U.S. Pat. No. 6,023,980, a column assembly is used with a conventional tensile testing machine to create the high-cycle forces. In particular, a laboratory test machine arrangement is described, in which an isolation mass, a dynamic actuator, and a specimen coupler are placed on both ends of a specimen. The specimen is mounted between opposing faces of the couplers. The result is a column assembly that is symmetrical on either side of the specimen. When mounted in a tensile testing machine, the column may be placed in tension to produce a desired static tensile stress in the specimen. The dynamic actuators may then be excited to produce axially oriented mechanical vibration resonances in the column assembly.